A light source for illuminating an information source is often required in many applications. In particular, liquid crystal displays (LCDs) have become more and more popular in many electronic media LCDs are commonly adopted in various applications, such as laptop computers, display monitors, video cameras, automatic teller machine displays, displays in avionics, televisions etc. In general, a backlight module is required for the LCDs to illuminate the information to be displayed. There are various kinds of light sources used in a backlight module of an LCD, e.g., fluorescent lamps and LEDs. While the fluorescent lamps are inexpensive and do not need complex control circuitry. They are sometimes inadequate for certain applications that require good color quality and long lamp life.
LEDs have been proposed for use as light sources, such as LCD backlight modules, for many reasons. These advantages of LED light sources include long life, ease of replacement, robust mechanical property, and better color quality than fluorescent lamps. Certain applications (e.g., avionics) require a specific chromaticity of light emitted from the LCD backlight module. However, most commercially available LEDs are made with a limited number of chromaticity choices and their chromaticity may change over time.
An LED light source with a raised LED 100, as shown in FIG. 1, to improve the chromaticity of a combined light was disclosed in U.S. Pat. No. 6,666,567. The raised LED 100 includes an LED diode 101 encased in a package 102 which is raised above the floor 103 of optical cavities. The raised structure permits light to be emitted from the base of the LED. Additionally, reflective protrusions may be placed beneath the raised LED to aid in redirecting the light trajectory. A combination of fluorescent lamps and LEDs were also proposed to form a hybrid light source. However, all these schemes increase the complexity and cost of the light source.
As shown in FIG. 2 and FIG. 3, an LCD backlight 200, which includes a first LED array 201 that provides light with a first chromaticity and a second LED array 202 that provides light with a second chromaticity, was disclosed in another U.S. Pat. No. 6,608,614. The lights emitted from these two LED arrays 201 and 202 are combined through a combining element 301 (e.g., a wave guide) and then projected towards an LCD stack 302. The LED chip normally emits light in a direction which is approximately perpendicular to the chip surface. The directions of light emitted from the first and the second LED arrays are approximately perpendicular and parallel to the panel surface, respectively. A separate combining element 301 is required in this light source. The chromaticity of the combined light can only be adjusted by changing the chromaticity of the second LED array 202 through a control system (not shown). Therefore, there is a limited flexibility for chromaticity adjustment.
According to another prior art, a Luxeon side-emitter having packaged LED chips was disclosed, as shown in FIG. 4. The side-emitter may provide good uniformity of combined light but the light intensity is poor. In addition, packaged LED chips normally occupy a much larger area than the bare chips scheme of the present invention. It is known that the majority of lights emitted from LED chips travel in a direction approximately perpendicular to the chip surface. Therefore, the LED chips need to be arranged in a way such that the lights emitted from different LED chips have a chance to be combined and mixed in order to achieve desired chromaticity before they reach a display screen.
In a conventional LED packaging structure shown in FIG. 5, an LED chip 500 is attached to a packaging substrate 511. The LED chip comprises a negative electrode (bonding pad) 501, an n-type cladding layer 503, an active layer 504, a p-type cladding layer 505, a semiconductor substrate layer (e.g., GaAs or GaN) 506, and a positive bonding electrode (bonding pad) 502. The negative electrode 501 of the LED chip 500 is connected through a gold or aluminum wire 512 to a negative bonding pad 513 on the packaging substrate 511, and the positive electrode 502 is soldered on a positive bonding pad 514 on the packaging substrate 511. The LED chip 500 and the gold wires 512 are then covered with a transparent resin 515 to isolate them from the outside environment. Only the metal pads or the connection pins 513 and 514 are left exposed for power source connection. The disadvantage of this LED structure is poor light intensity because the non-transparent metal pads 501 and 502 block a significant portion of the emitted lights. Moreover, the requirement of a conventional wire bonding process increases its process complexity, package size, and cost